The present invention relates to a process for the preparation of dinitramidic acid and salts thereof. In particular the present invention relates to a process for the preparation of dinitramidic acid and salts ammonium dinitramide (ADN)/potassium dinitramide using solid acid catalysts such as metal ion exchanged clay catalysts and surface supported catalysts which finds use in making high-energy solid propellants. Ammonium dinitramide (hereafter referred to as ADN) is considered a suitable replacement for ammonium perchlorate (AP) in solid propellant formulations. It is a good halogen free oxidizer with better performance characteristics for use in solid propellant applications.
The present invention also relates to a novel and an ecofriendly process for the preparation of dinitramidic acid by nitration of ammonium sulfamate with fuming nitric acid as a nitrating agent using solid acid catalysts selected from the group consisting of a montmorillonite clay catalyst and metal ion exchanged K10 montmorillonite clay catalysts and surface supported catalysts and preparation of ammonium dinitramide by neutralization of the dinitramidic acid with ammonia and separation of ammonium dinitramide by a conventional work-up procedure. The same process is also used for making potassium dinitramide, which is prepared by taking potassium sulfamate as the starting material and KOH as the neutralising agent.
Nitration is generally performed by the classical method employing the H2SO4xe2x80x94HNO3 system and also with strong nitrating agents such as NO2BF4 or N2O5.
Reference may be made to U.S. Pat. Nos. 5,316,749 and 5,316,749A, wherein ammonium salt of 1,1,3,3-tetraoxo-1,2,3-triazapropene anion (ADN) is prepared by the direct nitration of ammonia. Nitration is carried out at very low temperatures (xe2x88x9278xc2x0 C.). The formation of the product can be explained by the following equations:
NH3+NO2BF4xe2x86x92NH2NO2+HBF4xe2x80x83xe2x80x83(1)
NH2NO2+NO2BF4xe2x86x92HN(NO2)2+HBF4xe2x80x83xe2x80x83(2)
HN(NO2)2+2 HBF4+3 NH3xe2x86x92H4N(NO2)2 +2 N H4BF4xe2x80x83xe2x80x83(3)
The overall reaction is given in equation (4)
4NH3+2 NO2BF4xe2x86x92NH4N(NO2)2+2N H4BF4xe2x80x83xe2x80x83(4)
Reference may be made to U.S. Pat. Nos. 5,198,204 and 5,254,324, wherein the nitration of monoisocyanates of the type Zxe2x80x94CH2xe2x80x94CH2xe2x80x94NCO, where Z is an electron withdrawing group such as xe2x80x94CN, xe2x80x94COOR, or xe2x80x94COOH to the corresponding alkyl dinitramine is described. The nitration is carried out by using strong nitrating agents such as NO2BF4 or N2O5. The alkyl dinitramine gives ADN on treatment with ammonia. The drawbacks are these strong nitrating agents are very expensive and are not suitable for large-scale industrial production of oxidizers.
Reference may be made to U.S. Pat. Nos. 5,145,852 and 5,659,080 wherein the nitration of deactivated amines such as NH2NO2, NH2CONH2 or NH2COOC2H5 to dinitramidic acid using the nitrating agents such as NO2BF4 or N2O5 is described. The drawbacks are these strong nitrating agents are very expensive and are not suitable for large-scale industrial production of oxidizers. Most of the reactions are highly exothermic and involves the use of highly reactive nitrating agents and hence are to be carried out at very low temperatures (xe2x88x9230xc2x0 C. to xe2x88x9250xc2x0 C.) in small batches.
Reference may be made a publication by Bottaro et al, J.Am.Chem. Soc, 9305, 1997 wherein ammonium dinitramide is prepared by the reaction of nitramide, nitronium tetrafluoroborate and ammonia or ammonia with N2O5 or N2O5 and ammonium nitromethane.
Reference may be made to U.S. Pat. No. 5,976,483 wherein the preparation of dinitramidic acid by nitration of a compound selected from a group consist of NH2SO3H, NH(SO3H)2, N(SO3H)3 and its salts such as ammonium sulfamate using a common nitrating agent such as nitric acid/sulphuric acid or nitric acid/acetic anhydride is described. The invention also relates to a method of preparing dinitramide salt, comprising neutralization of the dinitramidic acid with ammonia, recovery and purification of the salt by passing through a column and then eluted by gradient elution.
2 HNO3+2 H2SO4+NH2SO3NH4xe2x86x92HN(NO2)2+2 H2SO4+NH4HSO4+H2Oxe2x80x83xe2x80x83(5)
HN(NO2)2+2NH3+NH4HSO4xe2x86x92NH4N(NO2)2+(NH4)2SO4xe2x80x83xe2x80x83(6)
The drawbacks are the use of hazardous sulfuric acid and environmental pollution during disposal of spent acid.
Reference may be made a publication by Malesa and Skupinski in Propellants, Explosives, Pyrotechnics, 24, 83-89, 1999 wherein the ammonium dinitramide is prepared by the method described by Bottaro et al in acetonitrile and the product is separated by evaporation of the neutralised solution to dryness after filtration, followed by extraction with isopropanol and evaporation to dryness and finally by recrystallization from ethyl acetate.
The main object of the present invention is to provide an ecofriendly method for preparing dinitramidic acid and its salts by nitration of ammonium sulfamate with fuming nitric acid as a nitrating agent using solid acid catalysts selected from the group consisting of a montmorillonite clay catalyst and metal ion exchanged K10 montmorillonite clay catalysts and surface supported catalysts and preparation of ammonium dinitramide by neutralization of the dinitramidic acid with ammonia and separation of ammonium dinitramide by a conventional work-up procedure which obviates the drawbacks as detailed above.
Another object of the present invention is the use of ecofriendly metal ion exchanged clays as solid acid catalysts in the nitration of ammonium sulfamate, dispensing the use of sulfuric acid which obviates the drawbacks as detailed above.
Another object of the present invention is to dispense the use of the nitrating agents such as NO2BF4 or N2O5 which obviates the drawbacks as detailed above.
The present invention provides an ecofriendly process for the preparation of dinitramidic acid by nitration of ammonium sulfamate with fuming nitric acid as a nitrating agent using solid acid catalysts selected from the group consisting of a montmorillonite clay catalyst and metal ion exchanged K10 montmorillonite clay catalysts and surface supported catalysts. Ammonium dinitramide is then prepared by the neutralization of the dinitramidic acid with ammonia and separation of ammonium dinitramide by a conventional work-up procedure.
Accordingly, the present invention relates to a process for the preparation of dinitramidic acid comprising nitrating ammonium sulfamate with fuming nitric acid as a nitrating agent using a solid acid catalyst selected from the group consisting of a montmorillonite clay catalysts, metal ion exchanged K10 montmorillonite clay catalysts and surface supported catalysts.
In one embodiment of the invention, the catalyst comprises a metal ion exchanged clay, with the metal ion preferably being selected from the group consisting of Fe3+, Al3+, La3+, Cu2+ and Zn2+.
In another embodiment of the invention, the surface supported catalyst comprises titanium or molybdenum supported on silica-alumina.
In another embodiment of the invention, the reaction is carried out in the presence or absence of solvent.
In another embodiment of the invention, the solvent is selected from carbon tetrachloride and dichloroethane.
In another embodiment of the invention, the nitration is carried out in the absence of sulfuric acid.
In another embodiment of the invention, the solid acid catalyst used act as bifunctional catalysts, generating electrophile nitronium ion as well as instant adsorbent for water formed during the reaction to facilitate electrophilic substitution.
The invention also relates to a process for the preparation of ammonium dinitramide comprising nitrating ammonium sulfamate with fuming nitric acid as a nitrating agent using a solid acid catalyst selected from the group consisting of a montmorillonite clay catalyst, metal ion exchanged K10 montmorillonite clay catalysts and surface supported catalysts, and neutralizing the dinitramidic acid obtained with ammonia as the neutralizing agent and separating ammonium dinitramide.
In one embodiment of the invention, the catalyst comprises a metal ion exchanged clay, with the metal ion preferably being selected from the group consisting of Fe3+, Al3+, La3+, Cu2+ and Zn2+.
In another embodiment of the invention, the surface supported catalyst comprises titanium or molybdenum supported on silica-alumina.
In another embodiment of the invention, the reaction is carried out in the presence or absence of solvent.
In another embodiment of the invention, the solvent is selected from carbon tetrachloride and dichloroethane.
In another embodiment of the invention, the nitration is carried out in the absence of sulfuric acid.
In another embodiment of the invention, the solid acid catalyst used act as bifunctional catalysts, generating electrophile nitronium ion as well as instant adsorbent for water formed during the reaction to facilitate electrophilic substitution.
In still another embodiment of the present invention, ammonium sulfamate (as such or dispersion in inert solvents) is nitrated with 100% nitric acid in the presence of metal ion exchanged clay catalysts and the reaction monitored by measuring the concentration of dinitramidic acid by UV spectroscopy.
Scientific Explanation
The principal object of the present invention by processing under the above conditions, therefore, was to produce cost effective ammonium dinitramide over the previous reported works. The other object of the present invention was achieved by metal ion exchanged clays as solid acid catalysts replacing the hazardous sulfuric acid or the nitrating agents such as NO2BF4 or N2O5 which are very expensive and are not suitable for large-scale industrial production of oxidizers. The novelty of the present invention with respective to the prior art is to produce dinitramidic acid or its salts by nitration of ammonium sulfamate using solid acid catalysts replacing sulfuric acid. Apart from generating nitronium ion, the solid acid catalysts, metal ion exchanged clays also adsorbs water formed in the reaction.